In recent years, a digital camera has come into wide use rapidly, and it has exceeded a device to take in simply an image into a computer, to be used widely as a tool to take photographs in the same way as in conventional silver halide cameras. Under the aforesaid condition, demands for functions useful for taking photographs (for example, a higher variable power ratio and a wider angle of view) and for providing a thin and compact digital camera have become strong. Further, achievement of higher optical ability is requested, because the number of pixels of the image pickup element is in the trend to increase year after year. Even an equipment for taking in a moving image, such as a camcorder, is requested an optical ability that is higher than the conventional one, with a background of coping with still image picture-taking function and with high definition television such as Hi-Vision.
There is generally used a technique to bend an optical path in an optical unit as an effective technique for reducing thickness of an apparatus having a function of image pickup, such as a digital camera. For example, Unexamined Japanese Patent Application Publication (JP-A) No. 2006-71993 suggests a zoom lens including, in order from the object side, a first lens group having positive optical power including a prism that bends an optical path, a second lens group having negative optical power, a third lens group having positive optical power, a fourth lens group having positive optical power and a fifth lens group. The zoom lens achieves variable power ratio of about ×5 by constituting the so-called positive-lead zoom type. Further, JP-A No. 2004-348082 suggests a zoom lens achieving a wider angle of view by including, in order from the object side, a first lens group having negative optical power including a prism that bends an optical path, a second lens group having negative optical power, and a third lens group having positive optical power.
As a zoom lens of a straight type in which the optical path is not bent, there has been suggested a zoom structure having a variable-power ratio that is higher than that of the zoom structure of folded optics type in which the optical path is bent. For example, JP-A Nos. 5-107476 and No. 5-323196 disclose a zoom lens attaining the variable-power ratio that is as high as about ×7-×17 by providing a first lens group having negative optical power, a second lens group having positive optical power, a third lens group having negative optical power, a fourth lens group having positive optical power and a fifth lens group having positive optical power in this order from the object side, and by moving the second lens group, the third lens group and the fifth lens group for varying power.
The zoom lens disclosed in JP-A No. 2006-71993 achieves a variable power ratio of about ×5 but has the total angle of view at the wide-angle end of about 60°, which is not regarded as a wide angle of view. When bending the optical path by arranging a reflection optical element in the first lens group, a size of a space needed to bend an optical path (when the reflection optical element is a prism for example, it corresponds to an optical path length of the prism) depends on a beam height that is highest among the off-axial beam height on an incident surface of a reflection optical element and the off-axial beam height on an emergent surface of the reflection optical element, on sections including optical axes before and after the optical path is bent. Therefore, for making a space needed for the bending to be small, it is effective to arrange a negative lens at the object side of the reflection optical element, and to position an entrance pupil position to be closer to the object side. In the zoom lens disclosed in JP-A No. 2006-71993, it is difficult to make negative optical power of the negative lens positioned on the object side of the reflection optical element to be stronger, because an optical power of the first lens group is positive. Therefore, when providing a wider angle of view with the zoom lens disclosed in JP-A No. 2006-71993, a space needed for the bending is reluctantly increased.
The zoom lens disclosed in JP-A No. 2004-348082 achieves a wide angle of view exceeding 700, but has a variable power ratio of about ×3, which is not regarded as sufficient. In a negative-lead zoom type wherein the first lens group has negative optical power, it is easy to make an effective diameter of a lens closer to the object to be small, because an entrance pupil is generally positioned to be relatively close to the object. In the negative-lead zoom type, and there is also a merit suitable for a wider angle of view such as it can have a structure of a retro-focus type easily at the wide-angle end. However, the zoom lens disclosed in JP-A No. 2004-348082 varies its power by moving greatly a lens group including a diaphragm and positive optical power, which causes a problem that F-number is largely fluctuated due to varying power under the condition that an open aperture of a diaphragm has a fixed diameter. Therefore, for achieving higher variable power while controlling fluctuations of F-number to be within an allowable range, the zoom lens requires a mechanism to change a diameter of a diaphragm when varying its power.
Though the zoom lens disclosed in JP-A Nos. No. 5-107476 and No. 5-323196 has achieved simultaneously an angle of view of about 70° and a variable-power ratio exceeding ×7, its capability is not sufficient to be used for a still camera. Since negative optical power of the first lens group is relatively weak, an effective diameter of the first lens group tends to be large, which is a problem, and a space necessary for bending an optical path is not secured. Therefore, it is difficult for these zoom lenses to realize a thinner image pickup apparatus.